In recent years, a variety of electronic devices such as organic semiconductors or light-emitting elements in which an organic compound which has photoelectric function as well as reversible oxidation-reduction characteristics and can form amorphous film in itself is used as an organo-electronic material, for example, as a hole transporting agent, have attracted considerable attention. Such an amorphous film of organic substances are formed by preparing a coating composition comprised of a binder resin such as polycarbonate resin and the organic compound dissolved in a suitable organic solvent and then by coating and drying the composition, as described in JP-A-11-174707. An organic amorphous film is also formed by preparing a coating composition comprised of a multinuclear aromatic tertiary amine called a “star-burst” compound dissolved in a suitable organic solvent and then by coating the composition on a substrate and drying it, as described in JP-A-08-291115.
According to a method using a binder resin among the methods mentioned above, the organic compound is diluted with the binder resin in the resulting amorphous film and influenced by the binder resin so that the organic compound cannot exhibit sufficiently the functions that it originally has as an organo-electronic functional material. In addition, if the organic compound forms an amorphous film that is stable at normal temperature with the aid of a binder resin, the organic compound has a low glass transition temperature so that the film is poor in heat resistance and has a problem in stability and life.
Under these circumstances, polynuclear aromatic tertiary amine compounds called the “star-burst” molecules are attracting considerable attention since they are capable of forming amorphous film in themselves at normal temperatures, as described above. The star-burst molecules are divided into three groups based on their molecular structures: compounds having a triphenylamine structure (triphenylamines), compounds having a triaminobenzene structure (triaminobenzenes) and compounds having a triphenylbenzene structure (triphenylbenzenes).
Examples of the triphenylamines include, for example, 4,4′,4″-tris-(N,N-diphenylamino)triphenylamine (TDATA) and 4,4′,4″-tris(N-phenyl-N-m-tolylamino)triphenylamine (m-MTDATA), as described in JP-A-01-224353, and in addition, 4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)triphenylamine (2-TNATA), as described in JP-A-08-291115.
These triphenylamines are reversible in oxidation-reduction process and can form amorphous film by a vapor deposition method, however, TDATA and m-MTDATA have a problem in heat resistance. TNATA has a glass transition temperature of about 110° C. and is superior in heat resistance, but it is readily crystallized so that the amorphous film formed therewith is lacking in stability.
Examples of the triphenylbenzenes include, for example, 1,3,5-tris(4-(N,N-diphenylaminophenyl)benzene (TDAPB) and 1,3,5-tris(4-(N-tolyl-N-phenylaminophenyl)benzene (MTDAPB), as described in Bando Technical Report, Vol. 2, pp. 9-18, 1998 (Bando Chemical Industries, Ltd.). These triphenylbenzenes also can form amorphous film and have oxidation potentials in the range of 0.6-0.7V, but they are irreversible in oxidation-reduction process so that they are not suitable for practical use as an organo-electronic functional material such as a hole transporting agent.
On the other hand, examples of the triaminobenzenes include 1,3,5-tris(N-methylphenyl-N-phenylamino)benzene (MTDAB). The triaminobenzenes also have oxidation potentials in the range of 0.6-0.7V, but they are irreversible in oxidation-reduction process so that they are also not suitable for practical use as an organo-electronic function material.
The present inventors have presented 1,3,5-tris(N-(p-methylphenyl)-N-(-1-naphthyl))aminobenzene (p-MTPNAB) and 1,3,5-tris(N-(p-methylphenyl)-N-(4-biphenyl)amino)benzene (p-MTPBAB) as such organic compounds that are reversible in oxidation-reduction process, have oxidation potentials in the range of 0.5-0.7V, are superior in heat-resistance, and can form amorphous film by a vapor deposition method, as described in Japanese Patent Application No. 2003-079441.
The above-mentioned p-MTPNAB and p-MTPBAB are reversible in oxidation-reduction process and have high oxidation potentials as well as high glass transition temperature, i.e., 87° C. and 98° C., respectively. However, when they are subjected to repeated oxidation-reduction process, peak currents of oxidation curves tend to fall, and accordingly, there is a fear that they have not enough stability and durability for use as organo-electronic functional material.
On the other hand, a hole transporting agent comprising a polynuclear aromatic tertiary amine compound having a biphenyl skeleton is also known. Examples of such polynuclear aromatic tertiary amine compounds include 4,4′-bis(N-(3-methylphenyl)-N-phenylamino)-biphenyl (TPD), as described in JP-A-07-090256, and 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (α-NPD), as described in JP-A-05-234681 However, an organic electroluminescence element manufactured by using these compounds as a hole transporting agent needs a high voltage to be driven.
The invention has been completed to solve the problem involved in the known organo-electronic functional materials as mentioned above. Therefore, it is an object of the invention to provide an organo-electronic functional material which has an opto-electronic function, is reversible in oxidation-reduction process, can form amorphous film in itself, and has a high glass transition temperature, and shows only a slight change of peak current when being subjected to repeated oxidation-reduction process, and is hence superior in stability.